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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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228                                ALASKAN GLACIER STUDIES
/ raphy of glacial deposits. Not merely are kettle and kame areas caused, but extensive depressions also result. The melting of Galiano Glacier, if further deposit should cease, would give rise to a depression of two or three square miles. Such buried blocks may last long, and receive a heavy burden of deposit, and their ultimate melting would give rise to an irregular topography, varying according to conditions, and depressions for lakes or ponds, with irregular bottoms and margins. In regions of former glaciation a multitude of kettles and pond and lake depressions exist whose interpretation is made clear by the assumption of the former presence of buried ice blocks. In our own work in such formerly-glaciated regions we have hitherto often hesitated to assume this hypothetical cause in the absence of direct proof; but with the evidence of the frequency and effectiveness of the buried ice blocks in the Alaskan region there is little reason for such hesitation. We are convinced that on the margin of waning ice sheets in a region of irregular topography, such as prevailed over extensive areas in Europe and America in the closing stages of the Glacial Period, the conditions favoring the burial of detached or partially detached ice blocks, and of stagnant glacier termini, were common, and that the results of then* subsequent melting are of great importance in determination of topographic detail.
The mountain history of the Alaskan coast region has been long and complex, with several periods of mountain growth and intervals of erosion, and with periods of volcanic activity. The details of this history are not yet worked out, and we have no knowledge as to the climatic conditions. During the Tertiary, and perhaps in the Pliocene division of the Tertiary, coal beds were accumulated, and associated with them are found plants of a warmer or wetter climate. From the topography of the foothills near the coast we infer that there was a long period of subaerial erosion, during which the mountains were greatly reduced and a system of broad valleys developed. This period of erosion, at least in its later portion, was presumably free from extensive glaciation. The mountains are still rising, as the frequent earthquakes testify, and in 1899 there was further notable uplift, in one section of the coast amounting to forty-seven feet. It is probable that the mountains owe their present great height mainly to geologically-recent uplift and that the extensive glaciation of the region is due to the rising of the mountains in recent times. Whether the extension of glaciation accompanied the uplift as Russell inferred, or whether it was preceded by a period of stream erosion in lofty mountains not yet glaciated is not clear; but we are inclined to the latter view because of the extensive series of branching valleys, presumably formed originally by rivers and now occupied by glaciers. All this early history is obscure and at present can be the subject only of hypothesis.
Without being able to assign a period to the beginning of glaciation, we know that the mountains ultimately became clothed in snowfields and the valleys filled with glaciers, and that the condition has lasted a very long time. Whether the glaciers came solely as a result of the rising of the mountains, or whether they were dependent upon a climatic change, giving heavy snowfall to a region already lofty, cannot be positively stated. Nor do we know the history of the initial spread of the glaciers, of what episodes of advance and recession, if any, accompanied it. Ultimately, however, the expansion of the glaciers extended so far as to push the glacier termini beyond the mountain front, filling the entire inlet with a great glacier flood, and pushing the glacier fronts out to the mouth of